Performance enhancement of passively Q-switched Nd:YVO4 laser using graphene–molybdenum disulphide heterojunction as a saturable absorber

Abstract

Two-dimensional (2D) materials, such as graphene and molybdenum disulphide (MoS2), are attractive for pulse laser applications owing to their exceptional nonlinear optical properties. In this paper, we fabricated three different saturable absorbers (SAs) based on plasma-enhanced chemical vapour deposition (PE-CVD) and liquid-phase exfoliation. The transmissivities of fabricated graphene, MoS2 and graphene–MoS2 SAs were 88.76%, 91.88% and 82.23% at 1064 nm, respectively, which were measured by a UV–Vis-NIR spectrophotometer. Their modulation depths on SiO2 substrate were measured to be 7.1%, 7.8% and 15.01% by the balanced synchronous twin-detector measurement system. Subsequently, these SAs (graphene SA, MoS2 SA, graphene and MoS2 SA, and graphene-MoS2 heterojunction SA) were placed in the same linear laser resonator, and their pulse output characteristics were systematically analysed and compared. When the absorbed pump power was 6.56 W, the heterojunction-based passively Q-switched Nd:YVO4 laser generated stable laser pulses with a pulse width of 180 ns and corresponding repetition rate of 640 kHz, which showed strongly enhanced saturable absorption properties than individual graphene (320 ns, 980 kHz), individual MoS2 (280 ns, 630 kHz), or graphene and MoS2 (248 ns, 760 kHz). And we attributed this mainly to rapid carrier relaxation induced by interlayer charge transfer in heterojunction because of type II band alignment. These results open up a possibility of actively regulating the saturable absorption properties of SAs by constructing heterojunctions with 2D materials.